1. Technical Field
The present invention relates to wireless communications and, more particularly, wideband wireless communication systems.
2. Related Art
Modem radio frequency (RF) transmitters for applications, such as cellular, personal, and satellite communications, employ digital modulation schemes, such as frequency shift keying (FSK) and phase shift keying (PSK), often in combination with Code Division Multiple Access (CDMA) communication. Some of these communication schemes, for example the 270.83 kbit/s binary Gaussian FSK employed in the Global System for Mobile Communications (GSM) cellular telephony standard, have constant envelopes. A constant-envelope modulation is an attractive solution for RF systems that allows using a nonlinear power amplifier (PA), thereby improving the efficiency of the system in the transmit mode. An example of such a system is the GSM/GPRS (General Packet Radio Service) standard that uses Gaussian Minimum Shift Keying (GMSK) modulation. In general, a constant-envelope modulation signal can be specified as:s(t)=A cos [ωct+φ(t)],where φ(t) contains the information of the signal. Since the envelope of the signal is constant, the transmitter architecture is not limited to a Cartesian topology and hence a few other architectures have been introduced. Among these, a “translational loop” (also known as “offset phase lock loop (PLL)”) is a popular architecture.
While constant-envelope systems allow nonlinear PAs and architectures such as translational loop, they do not make an efficient use of bandwidth. Therefore, to increase the data rate over a given bandwidth, other data-efficient modulations must be used. An example of such a system is the Enhanced Data rate GSM Evolution (EDGE) standard in which the data rate is three times as high as that of the GSM/GPRS standard while using the same bandwidth. In this case the modulated signal can be specified ass(t)=A(t)cos [ωct+φ(t)].
In order to benefit the advantages of a nonlinear PA and a translational loop, a polar transmitter architecture is used. In this architecture, the phase and the envelope information are separated in the baseband. Then, the phase information, cos[ωct+φ(t)], goes to a constant-envelope transmitter and the envelope of the signal, A(t), is used to modulate the PA by controlling the power of the PA. While this architecture seems to be a good solution, some practical issues limit the implementation. Among these, the delay mismatch between the phase and envelope is an important issue. Generally, delay through the phase path (translational loop) and the envelope path (mainly the power control of the PA) often are not the same effectively resulting in envelope and phase signals being slightly out of phase resulting in power spectral leakage in adjacent bands. These delay variations are partially the result of CMOS fabrication process variations, as well as temperature variations, that affect the analog circuitry of the signal paths slightly differently. In some prior works this mismatch is ignored, degrading the quality of the transmit signal. Ignoring delay mismatch in RF polar transmitters, however, is not an optimum solution.
The power spectrum emitted from an EDGE transmitter will not be ideal due to various imperfections in the RF transmitter circuitry that cause the delay mismatch as well as other problems. One quality measure of the EDGE standard that relates to the RF signal power spectrum is the so-called spectral mask requirement. This spectral mask represents the maximum allowable levels of the power spectrum as a function of frequency offset from the RF carrier in order for a given transmitter to qualify for EDGE certification. For example, at a frequency offset of 400 kHz (0.4 MHz), the maximum allowable emission level is −54 dB relative to the carrier (dBc).
Another RF transmitter quality measure of the EDGE standard is the modulation accuracy, which relates the RF transmitter modulation performance to an ideal reference signal. Modulation accuracy is stated in root-mean-square (RMS) and peak values and is specified in percentage format. For a given transmitter to qualify for EDGE certification, the RMS modulation error must be less than 9% and the peak modulation error must be less than 30%. Thus, to meet EDGE certification, a need exists to measure the frequency offset power spectrum and, if necessary, adjust the delay mismatch to reduce the frequency offset power spectrum.
There is a need, therefore, for a method and an apparatus that addresses the problems associated with delay mismatch while adhering to spectral mask requirements to improve performance of polar transmitters that are presently being designed.